I rebuilt the motor myself and she runs like a top. I bought Caroline with her stock engine in fine working order; I had built this new engine for my Mark VII LSC. She runs great, but anywhere under 1800rpm, she chugs so badly and starts to buck. I immediately have to downshift to bring the rpm up to what she likes and then she runs great.

I don't understand why with the B cam and the 3.73's she runs like this... Is there too much duration for the gear or too much gear for the lift? Is the engine pulling in too much air with the CAI, larger intakes, and GT40's that the stock 19# injectors aren't supplying enough fuel? IS it a combination of the two?

This engine has always done this in this car. Last week I started running only NON-ethanol fuel and while it seemed to make her run considerably better, she still chugs...

The other problem I am having is when driving, either accelerating or maintaining speed, you hear this bub...bub..bub...........buh-bub...through the exhaust and you can feel it, almost like a miss... I have new '93 cobra spark plugs (to match the GT40 head/plug specs), maybe it's the wires? Or maybe the coil isn't as strong as the engine needs, just like the injectors...?

Help!!! Please let me know what you think. I am trying to narrow down my next move... This feels like a game of chess...

SN Certified TechnicianFounding Member

It's not about just chances, it is about doing the diagnostic work....

Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.

The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
See AutoZone Part Number: 25886 , $10

Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

I had a lot of bucking issues after I put in my TFS H/C/I. Found a vacuum leak that helped a little but it wasnt until I got an actual dyno tune at TMS that all that went away.
I'm definitely no expert. When it came to my build up I was like a blind man in an orgy. Lots of feeling around trying to find what I needed.